Circuit sever for ppm focused traveling wave tubes



M. PHILLIPS Sept.- 9, 1969 CIRCUIT snvsn FOR PPM FOCUSED TRAVELING WAVE TUBES Filed Feb. 21, 1967 INVENTOR. ROBERT M. PHILLIPS United States Patent US. Cl. 315--3.5 6 Claims ABSTRACT OF THE DISCLOSURE Conventional circuit severs (zero gain RF absorption regions which permit modulated beams to pass therethrough and inhibit spurious oscillation problems by limiting circuit section gain) are either complex and bulky or inadequate with regard to PPM focused traveling wave tubes which require the smallest possible spacing between the focusing structure and circuit in order to maximize H-field-weight ratios. By utilizing a lossy coating on insulating circuit support members e.g. bars, rods in conjunction with a conductive metal intermediate coating section on the rods it is found that arcing and subsequent corrosion problems are eliminated in a compact design which is suitable for PPM (periodic permanent magnet) focused traveling wave tubes.

BRIEF SUMMARY OF THE INVENTION This invention relates in general to the field of high frequency electron discharge devices operable in the microwave spectrum and more particularly to devices of the PPM focused traveling wave type employing slow wave circuits supported within a vacuum envelope by dielectric support members and provided with a novel circuit sever design.

Conventional circuit severs used in traveling wave tubes employing dielectric supported slow wave circuits such as rod or bar supported helix, ring-and-bar, ring-loopcontra or cross-wound helix, etc., generally involve either sectioning the tube itself and positioning lossy absorption material in the sever region with metallic by-pass tabs to the vacuum envelope as shown in FIG. 2 by way of example or alternatively involve bulky attenuator-metalattenuator rings disposed about both circuit and support members such as taught in US. Patent No. 2,939,996 by K. E. Zublin et al. issued June 7, 1960. In the first instance multiple support sections involving both plural slow wave circuits and plural rod sections are typical and produce fabrication problems as well as increased cost. In the second instance the bulky nature of the rings used to form the sever section prohibits the use of this approach in PPM focused tubes if optimum magnet focusing efficiency is desired. The present invention represents an improvement over the aforementioned designs by eliminating both multiple sections and undue bulk. It involves the combination of a metallic band or coating disposed intermediate of a pair of lossy attenuation coatings all of which is disposed on the support members. The metal coating eliminates arcing and corrosion problems which can ultimately lead to extensive turn to turn shorting in the non-severed portions of the tube and provides an excellent bleed off path for intercepted beam current without permitting charge build-up between circuit and surrounding envelope which is a prime casual factor in the above mentioned arcing and corrosion problems encountered heretofore. The design taught herein permits extending the operational power levels of traveling wave tubes without encountering catastrophic tube failures without the necessity of going to bulky or costly designs such as discussed above.

It is therefore an object of the present invention to provide a traveling wave tube having an improved circuit sever.

A feature of the present invention is the provision of 21 PPM focused traveling wave tube incorporating a dielectric supported slow wave circuit having a metal-lossy circuit sever region formed as coatings on the dielectric support members.

Other features and advantages of the present invention will become more apparent upon a perusal of the followingspecification taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmentary sectional view partly in elevation of a PPM focused traveling wave tube incorporating the teachings of the present invention.

FIG. 2 is a partly schematic view of a prior art circuit sever design.

FIG. 3 is an enlarged sectional view of the circuit severed region of FIG. 1 taken along the lines 3-3 in the direction of the arrows.

FIG. 4 is an enlarged view of a modified sever region encompassed by lines denoted 44 in FIG. 3.

FIG. 5 is complete cross-sectional view of the circuit depicted in FIG. 3 taken along the lines 5-5 in the direction of the arrows.

FIG. 6 is sectional view of a dielectric supported slow wave circuit and vacuum envelope illustrating typical pitting and resultant deposition problems heretofore encountered.

DETAILED DESCRIPTION Turning now to FIG. 1 there is depicted a typical example of a PPM focused traveling wave tube 8 incorporating a dielectric supported ring-loop slow wave circuit assembly 9 disposed in fixed relationship within a metal vacuum envelope 10 as by a keyed-envelope deformation technique well known in the art. The traveling wave tube incorporates a circuit sever region 11 which extracts essentially all the RF wave energy while permitting the modulated beam to pass downstream unhindered. Since the traveling wave tube itself, both of the severed type as shown in FIG. 1 and the non-severed type is well known both in theory and practice and numerous literature references are available which describe the operation and design thereof, only a brief discussion of the known aspects of the tube depicted in FIG. 1 will be presented herein.

In brief, the tube depicted in FIG. 1 employs an electron gun or beam forming and projecting means 12 of any conventional design disposed at the upstream end portion thereof internally of the evacuated housing of the tube which in the particular tube type depicted in FIG. 1 includes an insulator section 13 disposed about the gun 12 and vacuum sealed to a main accelerating anode region 14 which in turn is vacuum sealed to an elongated tubular envelope 10 within which the slow wave circuit assembly 9 is disposed. The downstream end portion of the device is terminated in an electron collector region 15 which serves to absorb the spent beam electrons. The collector 15 can be of any known design and serves to complete the downstream region vacuum envelope for the device.

Electromagnetic wave energy to be amplified is introduced to the device via any suitable RF coupler mechanism such as coaxial coupler 16 and extracted after amplification at the downstream end portion of the device via any suitable RF coupler such as coaxial coupler 17. A PPM focusing system 18 is used to maintain beam geometry in a manner well known in the art. The PPM focusing system includes a plurality of axially polarized permanent magnets 19 such as of Alnico or any other conventional magnet material disposed along the axial extent of the tube in conjunction with a plurality of soft iron or the like high permeability pole pieces 20 to provide any desired beam focusing field strength along the tube axis in a manner well known in the art. In order to conserve on magnet weight and optimize the efiiciency of the PPM focusing assembly, it is imperative that the slow wave circuit be disposed as close as possible to the PPM focusing assembly since the field strength drops off exponentially from the pole pieces. This requirement dictates an integral pole piece design such as taught in copending US. patent application Ser. No. 304,457, filed Aug. 27, 1963 by I. Ruetz et al. and assigned to the same assignee as the present invention wherein the vacuum envelope for the tube main body region is actually formed by a series of non-magnetic spacers bonded to iron pole pieces. Alternatively, the simple metal envelope approach shown in FIG. 1 with the metal envelope 10 generally operated at ground potential is suitable to minimize bulk and spacing between pole pieces and circuit. The particular slow wave circuit 22 shown in FIG. 1 is a ring-loop circuit supported by dielectric rods 23 at best seen in FIG. 5. The ring-loop is a simple variation of the ring-and-bar circuit described in the US. Patent No. 2,937,311 with loops 24 replacing the conventional bars. The slow wave circuit 22 is merely illustrative of one of many suitable circuits which could advantageously be used in the tube depicted in FIG. 1 and which utilize dielectric support members, the most important of which are the helix, ring-bar, and contrawound helix circuits. Preferably, the two rod support shown in FIG. is used with the ring-loop circuit although 3 or more support rods could be used also which is typical for helix circuits in general. In order to obtain good thermal contact between the slow wave circuit assembly and the surrounding metal envelope, the indentations 25, 26 in both circuit 22 outer diameter and envelope inner diameter are cut to form pairs of diametrically opposed axially elongated slots with curvatures that match the cylindrical support rod curvatures for maximum thermal contact and overall assembly rigidity. The circuit assembly may be secured within the envelope 10 in any well known manner e.g. by applying elongated pressure along the envelope 10 length and deforming same it is possible to slide in the slow wave circuit assembly and then let the envelope relax and obtain a good pressure fit in a manner well known in the art.

Turning now to FIG. 2 there is depicted a typical prior art circuit sever wherein the vacuum envelope and circuit assembly are split into sections and tabs 30 such as of tantalum or the like are bonded between the circuit ends and the envelope 31, 32 section ends respectively to bleed off intercepted beam current as well as improving the RF isolation properties of the sever in conjunction with the Aquadag or the like lossy RF coatings 33 on support rods 34. The deficiencies of this design have been discussed previously. The particular sever approach taught by the present invention is best seen in FIGS. 3 and 4 and involves the deposition of a metal coating on each of the support rods to cover perhaps two or three periods of the circuit (turns, pitch, etc.). Any good conductive metal may be utilized in conjunction with standard alumina, sapphire, etc. dielectric support members. For example, an 80% molybdenum-20% manganese alloy painted on and fired in a furnace at around 1450 C. to 1500 C. for around minutes has been found satisfactory for producing good adherence for .5 to 1 mil thick metal coatings. Obviously, any other known metal to ceramic coating approaches may be used. The lossy RF absorbing material 41, 42, e.g. Aquadag, pyrolytic graphite, etc. is deposited on either side of the :metal band 40 or preferably as in FIG. 4 the lossy material 43 is disposed over the band to a thickness of around .5 mil or preferably less. An excellent approach which produces good results is to use chemical vapor deposited carbon (pyrolytic graphite) coatings. Good results have been obtained by bubbling H carrier gas through benzene with the rods maintained at around 1500 C. In five minutes good coatings of around .5 mil thickness were obtained. Suitable masking techniques can be used to form the tapered regions which reduce RF reflections. Any number of circuit lengths can be covered depending upon the degree of RF absorption needed, e.g., 5 or 6 circuit periods.

A major problem in high power traveling wave tubes using the above type slow wave circuit assemblies and any material making contact with one or more of the rods, envelope and circuit is the fact that electrostatic charges build up on the circuit due to beam interception at rates which simple RF lossy coatings cannot handle. This eventually results in arcing occurring between the lossy material and the circuit to an extent wherein metal from the circuit is actually drawn away and deposited on the lossy material or what is left of it since it will burn off quite fast under arcing conditions. This can and does result in circuit corrosion 48 lossy coating deterioration and even turn to turn shorting 49 as seen in FIG. 6 way beyond the bounds of the sever itself. These phenomena can cause catastrophic tube failure because they simply run away in an uncontrollable fashion once initiated. The present invention obviates all of these problems by the use of the metal band approach in conjunction with the lossy coatings on the support rods themselves. This approach provides e.g. a one-half ohm path for bleeding off intercepted beam current and thus eliminates arcing problems since the metal band 40 forms a DC. short between circuit 22 and metal envelope 10. The metal band approach also provides an RF short which has been found to introduce some 25 db and more insertion loss beyond what the lossy material alone will produce. The rods can also be brazed directly to the metal sever band if desired for added rigidity to minimize slippage problems. A simple pressure fit has been adequate to actually cause good metal band 40 to circuit contact directly through the thin pyrolytic graphite coating which can be deposited directly over the metallized band as shown in the modification in FIG. 4.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description and as shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A high frequency electron discharge device of the traveling wave type including electron beam forming and projecting means disposed at the upstream end portion of the device, electron beam collector means disposed at the downstream end portion of the device and elongated metallic vacuum envelope means disposed therebetween, said vacuum envelope means having a slow wave circuit assembly disposed therein for providing beam-wave interaction, said slow wave circuit assembly including a plurality of elongated dielectric support members disposed along the device axial length in intimate physical contact with an elongated slow wave circuit disposed interiorly of the support members and in intimate contact with said elongated vacuum envelope means disposed exteriorly of the support members, said slow wave circuit assembly including a circuit sever region which is a region of essentially zero gain and pure RF absorption which permits a modulated electron beam to pass therethrough, said circuit sever region including a conductive metallic coating on said support members in intimate contact with said metallic vacuum envelope means and with said slow wave circuit, said conductive metallic coating acting as a low resistance D.C. short between said elongated vacuum envelope means and said slow wave circuit and a lossy RF absorption coating disposed on said support members and lengths of said slow wave circuit on either side of saidband.

3. The device defined in claim 1 wherein said lossy RF absorption coating is pyrolytic graphite.

4. The device defined in claim 1 wherein said elongated vacuum envelope means disposed about said slow wave circuit is a metal tube and wherein a periodic permanent magnet focusing assembly is supported thereon in intimate contact therewith.

5. The device defined in claim 1 wherein said lossy RF absorption coating is tapered on either side of said conductive metallic coating and wherein said dielectric support members are elongated rods disposed in elongated slots having similar curvature in both said elongated vacuum envelope means and said slow wave circuit.

6. The device defined in claim 1 wherein both said conductive metallic coating and said lossy RF absorption coating make intimate physical contact with both the vacuum envelope means and the slow wave circuit and extend along a plurality of periodic lengths of said slow wave circuit.

References Cited UNITED STATES PATENTS 2,809,321 10/1957 Johnson et a1. 3153.6 2,911,599 11/1959 Klein et al. 3153.5 X 2,994,008 7/1961 Geiger et al 3153.5 3,050,657 8/1962 Branch 3l5---3.6 3,293,482 12/1966 Wolkstein 3153.6 X 3,368,103 2/1968 Thall 3153.5

HERMAN KARL SAALBACH, Primary Examiner SAXFIELD CHATMON, IR., Assistant Examiner US. Cl. X.R. 3l5-3.6 

